Electrode for electrochemical analyses

ABSTRACT

The properties of dental amalgam as an electrode material in voltammetry are presented. It was found that dental amalgam has a very high overpotential to hydrogen, allowing one to carry out trace analyses at potentials sufficiently negative to allow determination of e.g. zinc, cobalt, nickel and iron at trace levels. This has not previously been possible except from using a mercury or a mercury film electrode. Such determinations are very important for field and online analyses of pollutants in soil and groundwater, and the electrode can be used repeatedly.

[0001] The present invention relates to an electrode for use inelectrochemical analysis.

[0002] Since the development of polarography by Professor J. Heyrovskyin 1922, liquid mercury and liquid diluted mercury amalgams have beensuperior as electrode material for the use of voltammetry for analyticalpurposes. This Is mainly due to the high overvoltage to hydrogen, whichenables one to use a wide potential range for the measurements. Atypical example is the determination of zinc, this being virtuallyimpossible without using a mercury—or a mercury film electrode.

[0003] Due to the toxicity of mercury and liquid diluted mercurycompounds, the use of such compounds are increasingly restricted, andcannot be included in voltammetric devices for field and onlineapplications.

[0004] The properties of dental amalgam as an electrode material involtammetry have been studied. It was found that dental amalgam has avery high overpotential to hydrogen, allowing one to carry out traceanalyses at potentials sufficiently negative to allow determination ofe.g. zinc, cobalt, nickel and iron at trace levels. This has notpreviously been possible except from using a mercury or a mercury filmelectrode. Such determinations are very important for field and onlineanalyses of pollutants in soil and groundwater, and the electrode can beused repeatedly. The electrode is also solid.

[0005] Silver and mercury are main components of dental amalgam.However, commercial dental alloy has some tin, copper and zinc contentin order to improve the mechanical properties for dental use. A puresilver amalgam can be used to avoid interferences.

[0006] The electrodes can be produced using techniques well establishedin dental practice.

[0007] Due to the special properties of dental amalgam compared withmercury itself, its toxicity is very low, although its use in the mouthis somewhat disputed. As no increased amount of mercury is reported inthe groundwater close to cemeteries, the use of such small electrodesfor soil and groundwater analyses are obviously without any hazard.

[0008] The present analyses describe such electrodes and somepreliminary practical applications for trace heavy metal analyses, usingdifferential pulse stripping voltammetry. This enables one to determinee.g. zinc in the concentration level, less than 10 ppb. Furtherimprovements can obviously be obtained by optimising the composition ofthe alloy and the electrolyte, and by application of sound to theelectrode system. (Application of sound to an electrode system isdescribed in the inventors' Norwegian patent application number 19991814.)

[0009] In a first aspect of the invention there is provided an electrodefor use in electrochemical analysis, where the electrode comprisesamalgam in a solid stat

[0010] In a second aspect the invention provides a means for performingelectrochemical analyses involving a redox reaction at an electrodesurface, comprising an analysis cell, a system of electrodes arranged inthe analysis cell filled with a solution to be analysed producing ameasuring signal as a consequence of a redox reaction at the electrodes,wherein the measuring signal is a measure of the concentration of acomponent in the solution, and where at least one of the electrodescomprises amalgam In a solid state.

[0011] The amalgam may be any solid amalgam to be approved to be atleast in the same environmental safety level as the requirements fordental use. The electrode may be in the form of a ceramic rod enclosingthe amalgam, and with a copper wire for electrical connection. Theamalgam in the electrode may be made using methods familiar in dentaltechniques.

[0012] The electrode may be used as a measuring electrode involtammetric analysis, especially differential pulse anodic strippingvoltammetry.

[0013] The invention is stated in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described in thefollowing with reference to the accompanying drawings, where

[0015]FIG. 1 shows a three-electrode system;

[0016]FIG. 2 is a cross-sectional view of a solid electrode according toan embodiment of the invention;

[0017]FIG. 3 is a graph showing a voltammetric scan of a 30 years oldtooth filling in KNO₃ (0.1M) solution;

[0018]FIG. 4 is a voltammogram showing the detection of zinc on aworking electrode containing a non-zinc amalgam for dental use;

[0019]FIG. 5 is a graph showing peak current for zinc as a function ofconcentration for the experimental conditions in FIG. 4; and

[0020]FIG. 6 is a voltammogram of a simultaneous detection of zinc,cadmium and lead.

EXPERIMENTAL

[0021] The analyses were performed as differential pulse anodicstripping voltammetry, using a three-electrode system as shown inFIG. 1. The three electrodes in FIG. 1 is an indicator electrode, whichis the dental amalgam electrode, a counter electrode and a referenceelectrode. The three-electrode system is immersed in an electrolyte. Thethree-electrode system is connected to a potentiostat. The potentiostatis connected to a computer. The voltammetric equipment was made in ourlaboratories, and is a digital device that can perform all modes ofvoltammetry.

[0022] The counter electrode was a platinum wire and potentials weremeasured vs. a silver/silver chloride/saturated silverchloride/saturated potassium chloride reference electrode.

[0023] The working electrode was a silver amalgam electrode asillustrated in FIG. 2. Three types of dental amalgams were investigated,

[0024] a. The initial experiments were performed by using an about 30years old amalgam tooth filling, attached to a copper wire with silverepoxy, and than sealed in cemit (from T+F GmbH). The copper wire wasconnected to the voltammetric equipment. These experiments were carriedout to study if the dental amalgam showed overpotential to hydrogen Theanalyses were performed by differential pulse anodic strippingvoltammetry in KNO₃ (0.1 M, 100 ml) solution that was purged withnitrogen (10 min) prior to the analyses.

[0025] b. The second silver amalgam electrode was prepared by thefollowing procedure using the facilities in a dental clinic, Equalamounts of analytical grade mercury were mixed with high coppernon-gamma 2—alloy for dental use (ARDENT FUTURA, AB Ardent, MarstaSweden). The non-gamma 2 alloy consists of 44.5% silver, 30.0% tin and25.5% copper. A Dentomat 2 Degussa amalgam mixer for dental use wasemployed for mixing the amalgam. The am Igam was forced into an inertceramic rod by an amalgam gun (No. 940, Hawe Neos Dental), in a waysimilar to the technique used for dental fillings, and sealed around acopper wire as shown in FIG. 2. The copper wire was connected to thevoltammetric equipment. The experiments using this electrode were doneto prove if it was possible to detect zinc at a non-zinc dental amalgamelectrode. If it was found that zinc could be deposited and detectedvoltammetrically by using this electrode, this would also indicate thatother heavy metals could be detected by use of a pure silver dentalamalgam. The analyses were performed as differential pulse anodicstripping voltammetry in NH₄Ac (005 M, 100 ml) solution. The solutionwas purged with nitrogen (10 min) prior to the analyses to avoid anydisturbance of oxygen.

[0026] c. The third silver amalgam electrode was prepared by mixing onepart of pure silver crystals for dental use (particle<50μ with 0.65parts analytical grade mercury, using a mortar. Immediately after mixingthe amalgam was pressed into a ceramic rod with an amalgam gun asdescribed above. Analyses were performed by differential pulse anodicstripping voltammetry in NH₄Ac (0.05 M. 100 ml) These analyses were doneto prove if it was possible to detect heavy metals, for instance zinc,cadmium and lead, on the dental amalgam electrode using a silver amalgamfree from impurities like copper and zinc.

[0027] The three types of working electrodes were polished once beforethe experiments using a fine soften sandpaper, and washed in waterpurified by Millipore Elix and then with Millipore Milli-Q Gradientsystem (Millipore Corporation, SA 67120 Molsheim France). Also standardsolutions were made by use of water from this water purification system.All reagents were of analytical reagent grade quality.

[0028] Results

[0029] The result will be divided into three sections, dealing with therespective working electrode used.

[0030] 3.1. Initial Results from Using the 30 Years Old Tooth Filing asWorking Electrode.

[0031] Some voltammetric scans were performed initially to find if thedental amalgam showed overvoltage to hydrogen to allow the use of a widepotential range. As explained under the experimental section a thirtyyears old tooth filling was used as a working electrode in avoltammetric arrangement. The typical voltammogram obtained in a KNO₃(0.1 M, 100 ml) solution is showed in FIG. 3. The voltammetric scan inFIG. 3 shows current (100 nA) as a function of potential (mV) and wasperformed with a scan rate of 10 mV/sec and pulse height 25 mV.

[0032] As shown in FIG. 3, there are no doubt about that the dentalamalgam possess the property of a greater overvoltage against hydrogen.The peaks observed are reflecting the composition of the tooth filling,presumably corresponding to zinc, tin, copper and silver, from left toright. If the scan had been extended to around +300 mV, a peak formercury would probably also appear.

[0033] 3.2. Results from Analyses Using a Non-Zinc (High CopperNon-Gamma 2-Alloy) Amalgam for Dental Use as Working Electrode.

[0034] Analyses in order to prove if it was possible to detect heavymeals on an electrode consisting of a non-zinc (high copper non-gamma2-alloy) amalgam for dental use was performed. Analyses of zinc in therange from 100 μg/l to 1500 μg/l in NH₄Ac (0.05 M) solution was carriedout. Differential pulse anodic stripping voltammetry with a scan rate of10 mV/sec, pulse height 70 mV, and a 120 seconds deposition time wasused.

[0035]FIG. 4 shows a voltammogram were zinc was added successively tothe NH₄Ac solution. In FIG. 4 current (100 nA) is shown versus potential(mV).

[0036] As seen in FIG. 4 the addition of zinc resulted in correspondingincrease in the current signal. FIG. 5 shows a plot of the peak current(μA) as a function of zinc concentration (μg/l). The peak current iscorrected for a 1.5 μA offset. As shown, there is a good linearity(R²=0.99) between the to variables The experimental conditions were asin FIG. 4.

[0037] The results obtained in this section prove that zinc can bedetected on the dental amalgam electrode. These results also indicatethat an analogous situation will appear for other heavy metals, likecadmium, lead and copper.

[0038] The reproducibility was investigated by performing two parallelsand is given in Table 1. Table 1 is analyses of zinc on dental amalgamelectrode, two parallels. The table state the current values (correctedfor offset) obtained in each parallel Also the average response, thestandard deviation and the relative standard deviation are listed in thetable. TABLE 1 Concen- Resp R tration ns sponse Avg. resp. Std.deviation Relativ std. μg/l μA μA μA μA deviation (%) 500 12.1 11.0 11.60.8 6.9 1000 22.8 20.3 21.6 1.7 8.0 1500 35.7 32.2 33.9 2.5 7.3

[0039] As seen in Table 1 there is a relative standard deviation ofabout 7.5%. This is good with a view to that the electrode surface ofthe working electrode only was polish with soft sandpaper.

[0040] 3.3. Results from Analyses Using a Dental Amalgam with PureSilver only as a Working Electrode in Voltammetry.

[0041] Some analyses were performed to investigate the use of a puresilver amalgam as a working electrode material. Some simultaneousanalyses of zinc, cadmium and lead were performed. The given metals arein the range from 100 μg/l to 150 μg/A in NH₄Ac (0.05 M) solution wasused. Differential pulse anodic stripping voltammetry with a scan rateof 10 mV/sec, pulse height 100 mV, and 180 seconds deposition time wasapplied. A typical plot of the given voltammogram with current (μA) as afunction of potential (mV) is shown in FIG. 6 for the simultaneousdetection of 150 μg/l zinc (Zn), 100 μg/l cadmium (Cd) and 100 μg/l lead(Pb) is plotted.

[0042] Conclusions.

[0043] From the given results it is found that a working electrodeconsisting of dental amalgam can be used in voltammetry for detection ofheavy metals and other species in ppb levels. Further improvements canobviously also be obtained by optimising the composition of the alloyand the electrolyte, and by application of sound to the electrode system(Ref. Patent application 1999 1814, Norway).

[0044] Dental amalgam has many properties making it a preferablematerial for electrode use in voltammetry. It has a high overvoltageagainst hydrogen and this enables one to determine several compoundswhich previously could not be determined except from using a mercuryelectrode. It is a solid material, and this makes it possible to be us din online analyses in the field. It can also be used repeatedly over along period of time without any maintenance, which is essential foronline and field apparatus.

[0045] The toxicity of dental amalgam is not greater than that peoplecan live with it inside their mouth for a whole life without anypoisoning effects, and its use is without any formal environmentalrestrictions Also in an electrode the amount of an amalgam will be muchlower than the amount used In a tooth filling. Moreover as no increasedamount of mercury is reported in the groundwater close to cemeteries,the use of such small electrodes for soil and groundwater analyses areobviously without any hazard.

[0046] Finally, it is easy and cheap to manufacture such electrodes,using techniques well established for dental clinics.

[0047] Having described preferred embodiments of the invention it willbe apparent to those skilled in the art that other embodimentsincorporating the concepts may be used. These and other examples of theinvention illustrated above are intended by way of example only and theactual scope of the invention is to be determined from the followingclaims

[0048] Acknowledgement

[0049] The authors are very grateful to dentist Thor Pande, Trondheim,Norway, for assisting with the dental techniques for production ofelectrodes in his dental clinic.

1. Electrode for use in electrochemical analysis, characterized in thatthe electrode comprises amalgam in a solid state.
 2. Electrode accordingto claim 1, characterized in that the amalgam is amalgam approved fordental use.
 3. Electrode according to claim 1, characterized in that theamalgam is any solid amalgam to be approved to be at least on the sameenvironmental safety level as the requirements for dental use. 4.Electrode according to claim 1, characterized in that the electrodecomprises a ceramic rod enclosing the amalgam, and a copper wire forelectrical connection.
 5. Electrode according to claim 1, characterizedin that the amalgam in the electrode is made of an amalgam initiallyformed as a paste, succeeded by a solidifying process.
 6. Electrodeaccording to claim 1, characterized in that the amalgam in the electrodeIs made using methods familiar in dental techniques.
 7. Means forperforming electrochemical analyses involving a redox reaction at anelectrode surface, comprising an analysis cell, a system of electrodesarranged in the analysis cell filled with a solution to be analysedproducing a measuring signal as a consequence of a redox reaction at theelectrodes, wherein the measuring signal is a measure of theconcentration of a component in the solution, characterized in that atleast one of the electrodes comprises amalgam in a solid state. 8.Method for performing voltammetric analyses, characterized by using anelectrode system comprising at least one electrode of dental amalgam orrelated solid amalgams with similar or less environmental hazard thanapproved amalgams for dental use, thereby increasing the applicablevoltage range.
 9. Use of the electrode according to claim 1 as ameasuring electrode in voltammetric analysis.
 10. Use according to claim9, characterized in that the voltammetric analysis is of the typedifferential pulse anodic stripping voltammetry.